The titular paraphrasing of the famous falling tree in the forest riddle was well and truly answered this week, and shows just how far satellite remote sensing has come in recent years.

Last week sometime between Monday 10th July and Wednesday 12th July 2017, a huge iceberg was created by splitting off the Larsen C Ice Shelf in Antarctica. It is one of the biggest icebergs every recorded according to scientists from Project MIDAS, a UK-based Antarctic research project, who estimate its area of be 5,800 sq km and to have a weight of more a trillion tonnes. It has reduced the Larsen C ice Shelf by more than twelve percent.

The iceberg has been named A68, which is a pretty boring name for such a huge iceberg. However, icebergs are named by the US National Ice Centre and the letter comes from where the iceberg was originally sited ‚Äď in this case the A represents area zero degrees to ninety degrees west covering the Bellingshausen and Weddell Seas. The number is simply the order that they are discovered, which I assume means there have been 67 previous icebergs!

After satisfying my curiosity on the iceberg names, the other element that caught our interest was the host of Earth observation satellites that captured images of either the creation, or the newly birthed, iceberg. The ones we‚Äôve spotted so far, although there may be others, are:

ESA‚Äôs Sentinel-1 has been monitoring the area for the last year as an iceberg splitting from Larsen C was expected. Sentinel-1‚Äôs SAR imagery has been crucial to this monitoring as the winter clouds and polar darkness would have made optical imagery difficult to regularly collect.

Whilst Sentinel-1 was monitoring the area, it was actually NASA‚Äôs Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the Aqua satellite which confirmed the ‚Äėbirth‚Äô on the 12th July with a false colour image at 1 km spatial resolution using band 31 which measures infrared signals. This image is at the top of the blog and the dark blue shows where the surface is warmest and lighter blue indicates a cooler surface. The new iceberg can be seen in the centre of the image.

Longwave infrared imagery was also captured by the NOAA/NASA Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite on July 13th.

Similarly, NASA also reported that Landsat 8 captured a false-colour image from its Thermal Infrared Sensor on the 12th July showing the relative warmth or coolness of the Larsen C ice shelf ‚Äď with the area around the new iceberg being the warmest giving an indication of the energy involved in its creation.

Finally, Sentinel-3A has also got in on the thermal infrared measurement using the bands of its Sea and Land Surface Temperature Radiometer (SLSTR).

ESA‚Äôs Cryosat has been used to calculate the size of iceberg by using its Synthetic Aperture Interferometric Radar Altimeter (SIRAL) which measured height of the iceberg out of the water. Using this data, it has been estimated that the iceberg contains around 1.155 cubic km of ice.

The only optical imagery we‚Äôve seen so far is from the DEMIOS1 satellite which is owned by Deimos Imaging, an UrtheCast company. This is from the 14th July and revealed that the giant iceberg was already breaking up into smaller pieces.

It‚Äôs clear this is a huge iceberg, so huge in fact that most news agencies don‚Äôt think that readers can comprehend its vastness, and to help they give a comparison. Some of the ones I came across to explain its vastness were:

Size of the US State of Delaware

Twice the size of Luxembourg

Four times the size of greater London

Quarter of the size of Wales ‚Äď UK people will know that Wales is almost an unofficial unit of size measurement in this country!

Has the volume of Lake Michigan

Has the twice the volume of Lake Erie

Has the volume of the 463 million Olympic-sized swimming pools; and

My favourite compares its size to the A68 road in the UK, which runs from Darlington to Edinburgh.

This event shows how¬†satellites are monitoring the planet, and the¬†different ways we can see the world changing.

Andros Island in The Bahamas. Acquired by Landsat 8 in February 2017. Data courtesy of NASA.

Blue holes are deep marine caverns or sinkholes which are open at the surface, and they get their name from their apparent blue colour of their surface due to the scattering of the light within water. The often contain both seawater and freshwater, and in their depths the water is very clear which makes them very popular with divers.

The term ‚Äėblue hole‚Äô first appeared on sea charts from the Bahamas in 1843, although the concept of submarine caves had been described a century earlier (from Schwabe and Carew, 2006). There are a number of well-known blue holes in Belize, Egypt and Malta amongst others. The Dragon Hole in the South China Sea is believed to be the deepest blue hole with a depth of 300 metres.

The Andros Island in The Bahamas has the highest concentration of blue holes in the world, and last week we watched a television programme called River Monsters featuring this area. The presenter, Jeremy Wade, was investigating the mythical Lusca, a Caribbean sea creature which reportedly attacks swimmers and divers pulling them down to their lairs deep within of the blue holes. Jeremy fished and dived some blue holes, and spoke to people who had seen the creature. By the end he believed the myth of the Lusca was mostly likely based on a giant octopus. Whilst this was interesting, by the end of the programme we were far more interested in whether you could see blue holes from space.

The image at the top is Andros Island. Although, technically it‚Äôs an archipelago, it is considered as a single island. It‚Äôs the largest island of The Bahamas and at 2,300 square miles is the fifth largest in the Caribbean. There are a number of well known blue holes in Andros, both inland and off the coast, such as:

Blues in the Blue Hole National Park on the Andros Island in The Bahamas. Acquired by Landsat 8 in February 2017. Data courtesy of NASA.

Blue Holes National Park covers over 33,000 acres and includes a variety of blue holes, freshwater reservoirs and forests within its boundaries. The image to the right covers an area of the national park. In the centre, just above the green water there are five black circles ¬†‚Äď despite the colour, these are¬†blue holes.

Uncle Charlie’s Blue Hole, also called Little Frenchman Blue Hole, is just off Queen‚Äôs Highway in Nicholls Town and has a maximum depth of 127 metres.

Atlantis Blue Hole has a maximum depth of about 85 metres.

Stargate Blue Hole his blue hole is located about 500 miles inland from the east coast of South Andros on the west side of The Bluff village.

Guardian Blue Hole is in the ocean and is believed to have the second deepest cave in The Bahamas, with a maximum explored depth of 133 metres.

Blue hole in the south of Andros Island in The Bahamas. Acquired by Landsat 8 in February 2017. Data courtesy of NASA.

The image to the¬†right is from the south of the island. Just off the¬†centre, you can see a blue hole surrounded by forests and vegetation.

So we can confirm that the amazing natural features called blue holes can be seen from space, even if they don‚Äôt always appear blue!

Monitoring fires from space has significant advantages when compared to on-ground activity. Not only are wider areas easier to monitor, but there are obvious safety benefits too. The different ways this can be done have been highlighted through a number of reports over the last few weeks.

Firstly, NASA have released images from different instruments, on different satellites, that illustrate two ways of how satellites can monitor fires.

Acquired on the 25 April 2017, an image from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite showed widespread fire activity across the Yucat√°n Peninsula in South America. The image to the right is a natural colour image and each of the red dots represents a point where the instrument‚Äôs thermal band detected temperatures higher than normal.

False colour image of the West Mims fire on Florida/Georgia boundary acquired by MODIS on 02 May 2017. Data courtesy of NASA. NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response.

Compare this to a wildfire on Florida-Georgia border acquired from NASA‚Äôs Aqua satellite on the 02 May 2017 using the Moderate Resolution Imaging Spectroradiometer (MODIS). On the natural colour image the fires could only be seen as smoke plumes, but on the left is the false colour image which combines infrared, near-infrared and green wavelengths. The burnt areas can be clearly seen in brown, whilst the fire itself is shown as orange.

This week it was reported that the Punjab Remote Sensing Centre in India, has been combining remote sensing, geographical information systems and Global Positioning System (GPS) data to identify the burning of crop stubble in fields; it appears that the MODIS fire products are part of contributing the satellite data. During April, 788 illegal field fires were identified through this technique and with the GPS data the authorities have been able to identify, and fine, 226 farmers for undertaking this practice.

Imaged by Sentinel-2, burnt areas, shown in shades of red and purple, in the Marantaceae forests in the north of the Republic of Congo.Data courtesy of Copernicus/ESA. Contains modified Copernicus Sentinel data (2016), processed by ESA.

Finally, a report at the end of April from the European Space Agency¬†described how images from Sentinel-1 and Senintel-2 have been combined to assess the amount of forest that was burnt last year in the Republic of Congo in Africa ‚Äď the majority of which was in Marantaceae forests. As this area has frequent cloud cover, the optical images from Sentinel-2 were combined with the Synthetic Aperture Radar (SAR) images from Sentinel-1 that are unaffected by the weather to offer an enhanced solution.

Sentinel-1 and Sentinel-2 data detect and monitor forest fires at a finer temporal and spatial resolution than previously possible, namely 10 days and 10 m, although the temporal resolution will increase to 5 days later this year when Sentinel-2B becomes fully operational.¬† Through this work, it was estimated that 36 000 hectares of forest were burnt in 2016.

Given the danger presented by forest fires and wildfires, greater monitoring from space should improve fire identification and emergency responses which should potentially help save lives. This is another example of the societal benefit of satellite remote sensing.

Last week the Pixalytics name got lifted towards space! In a previous blog we described how we were supporting the Plymouth University Space Society launching a weather balloon.

After a number of attempts were thwarted by the wind and weather patterns of Plymouth, last Friday was the big day. A small band of the Space Society pioneers alongside myself and Howard from Salcombe Gin, spent half an hour battling to control a weather balloon in the wind as it was pumped full of gas and had a small Pixalytics branded payload attached including a Go-Pro Camera, balloon locator, various battery packs and a small bottle of Salcombe Gin. At the top of the blog is an image of the gin high above Plymouth.

Once we were ready, the balloon was carefully walked back a few paces, and then with our hearts in our mouths, it was launched! We watched it rise gloriously until it disappeared into the low cloud that was covering the city. For anyone who wants to see the launch, it was filmed and streamed on Facebook and the recording can be found here.

Once the launch euphoria had subsided, the Space Society team jumped into a car to follow the balloon towards the predicted landing site of Taunton. The payload had a device inside which when called replied with the balloon‚Äôs location to enable progress to be tracked. The balloon actually ended up around thirty miles to the east of the prediction, coming to rest back on Earth in Yeovil. Once they got close, the team had to ask an elderly resident for permission to look through her garden for the payload package. However, it was a success and the payload was retrieved!!

On examination of the footage, sadly the Go-Pro seemed to malfunction about 15 minutes into the flight and therefore we were not able to get full flight footage. However, this is the space industry and not everything goes to plan. Once you launch most things are out of your hands!

From the flight length and distance travelled the Space Society team estimate that the balloon went up above 32,000 m. Whilst that is only about one third of the way to the Karman line, which sits around 100,000m and is commonly viewed as the boundary between the Earth‚Äôs atmosphere and the outer space, it‚Äôs probably the highest point the Pixalytics name will ever get!

Readers will be aware that we do like the unusual marketing opportunity. We‚Äôve previously had our name going at 100 miles per hour aboard a Caterham Formula One car, so who knows what might be next?

It was great to support local students with their adventure towards space, and hopefully it will inspire them to get a job in our industry and develop their own space career!

Animation showing the creation of islands in Dubai between 2001 & 2009 using Landsat images. Data courtesy of NASA.

This week we‚Äôre focusing on the development of Dubai‚Äôs land-coast interface between July 2001 and October 2009, looking specifically at the creation of the Palm islands and the World Archipelago. Dubai is the most populous city in the United Arab Emirates, home to 2.7 million people as of January 2017. In a place where Dubai police vehicles include a Lamborghini and a Ferrari, and where it‚Äôs possible to buy gold bars from vending machines perhaps it‚Äôs not surprising to see the creation of extravagant islands.

Palm Islands & The World Archipelago

In the animation at the top of the blog, the development of the Palm Islands and The World Archipelago are clearly visible. The first island created was Palm Jumeirah, the smallest of the three planned palm islands, and can be seen just off centre on the animation. It consists of a tree trunk, a crown with seventeen fronds and a surrounding crescent, and is approximately 25 square kilometres in size. Construction began in 2001 and was completed in 2006. The workers used GPS signals to determine the correct place to deposit sand to create the palm effect.

Built in tandem were the Palm Jebel Ali and The World Archipelago. Construction began in 2002 and was expected to be completed in 2015, however work stopped in 2008 due to the financial crisis. Work has remained suspended on Palm Jebel Ali, but development on the World may be about to start. The World has three hundred islands reclaimed from the sea, but most of them are bare sand. In the last twelve months there have been rumours that ‚ÄėThe Heart of Europe‚Äô project and floating seahorses around St Petersburg island could be developed in the near future.

It is also possible to see the preliminary creation of Palm Deira at the top of the animation. 300 million cubic metres of sand were used to form the initial reclamation. However, between 2009 and 2016 there has been no further development.

Images of Dubai in 2001, left, and 2009 taken by Landsat 7. Data courtesy of NASA.

It is also worth noting the significant urban sprawl between the first and last images. Dubai‚Äôs population increased by 95%, from 910,336 to 1,770,978, during the period we‚Äôre looking at and whilst the growth of Dubai is obvious, it is particularly visible¬†southeast of the Palm Jumeirah development.

Creating the Time Series Animation

The animation was created using the first (blue) visible band of the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) instrument. In May 2003, the scan line corrector ‚Äď used to compensate for forward motion of the spacecraft, ensuring scan lines are parallel ‚Äď failed. Consequently, the instrument images in a zigzag fashion; some data is captured twice, whilst some is not captured at all. As a result, 22 % of data in Landsat 7 images post May 2003 are missing. To compensate for this we‚Äôve used a Geospatial Data Abstraction Library (GDAL) tool to fill ‚Äúno data‚ÄĚ regions by interpolating from nearby valid pixels. The results, whilst not perfect, are nearly indistinguishable at this resolution.

Viewed from space, both the speed and scale of the development is mesmerising. It is no surprise that tourism is a vitally important part of the local economy, attracting more than 13 million visitors in 2014. With the limitations of available land in Dubai, developments are sure to start again.

Image of Yemen acquired by Sentinel-2 in August 2015. Data courtesy of ESA.

This blog post is about what I did and what thoughts came to mind on my three-month long ERASMUS+ internship at Pixalytics which began in July and ends this week.

During my first week at Pixalytics, after being introduced to the Plymouth Science Park buildings and the office, my first task was to get a basic understanding of what remote sensing is actually about. With the help of Sam and Andy‚Äôs book, Practical Handbook of Remote Sensing, that was pretty straightforward.

As the words suggest, remote sensing is the acquisition of data and information on an object without the need of being on the site. It is then possible to perform a variety of analysis and processing on this data to better understand and study physical, chemical and biological phenomena that affect the environment.

Examples of programming languages: C, Python & IDL

I soon realized that quite a lot of programming was involved in the analysis of satellite data. In my point of view, though, some of the scripts, written in IDL (Interactive Data Language), were not as fast and efficient as they could be, sometimes not at all. With that in mind, I decided to rewrite one of the scripts, turning it into a C program. This allowed me to get a deeper understanding of satellite datasets formats (e.g. HDF, Hierarchical Data Format) and improve my overall knowledge of remote sensing.

While IDL, a historic highly scientific language for remote sensing, provides a quick way of writing code, it has a number of glaring downsides. Poor memory management and complete lack of strictness often lead to scripts that will easily break. Also, it‚Äôs quite easy to write not-so-pretty and confusing spaghetti code, i.e., twisted and tangled code.

Writing C code, on the other hand, can get overly complicated and tedious for some tasks that would require just a few lines in IDL. While it gives the programmer almost full control of what‚Äôs going on, some times it‚Äôs just not worth the time and effort.

Instead, I chose to rewrite the scripts in Python which I found to be quite a good compromise. Indentation can sometimes be a bit annoying, and coming from other languages the syntax might seem unusual, but its great community and the large availability of modules to achieve your goals in just a few lines really make up for it.

It was soon time to switch to a bigger and more complex task, which has been, to this day, what I would call my ‚Äúmain task‚ÄĚ during my time at Pixalytics: building an automated online processing website. The website aspect was relatively easy with a combination of the usual HTML, Javascript, PHP and CSS, it was rewriting and integrated the remote sensing scripts that was difficult. Finally all of those little, and sometimes not quite so little, scripts and programs were available from a convenient web interface, bringing much satisfaction and pride for all those hours of heavy thinking and brainstorming. Hopefully, you will read more about this development in the future from Pixalytics, as it will form the back-end of their product suite to be launched in the near future.

During my internship there was also time for events inside the Science Park such as the Hog Roast, and events outside as well when I participated at the South-West England QGIS User Group meeting in Dartmoor National Park. While it is not exactly about remote sensing, but more on the Geographic Information System (GIS) topic it made me realize how much I had learned on remote sensing in my short time at Pixalytics, I was able to exchange my opinions and points of view with other people that were keen on the subject.

A side project I‚Äôve been working on in my final weeks was looking at the world to find stunning, interesting (and possibly both) places on Earth to make postcards from – such as one at the top of the blog. At times, programming and scientific research reads can get challenging and/or frustrating, and it‚Äôs so relaxing to just look at and enjoy the beauty of our planet.

It is something that anyone can do as it takes little knowledge about remote sensing. Free satellite imagery is available through a variety of sources; what I found to be quite easy to access and use was imagery from USGS/NASA Landsat-8 and ESA Sentinel-2. It is definitely something I would recommend.

Finally, I want to say ‚Äúthank you‚ÄĚ to Sam and Andy, without whom I would have never had the opportunity to get the most out of this experience, in a field in which I‚Äôve always been interested into, but had never had the chance to actually get my hands on.

Blog written by Davide Mainas on an ERASMUS+ internship with Pixalytics via the Tellus Group.

Did you know it‚Äôs World Space Week? It occurs between the 4th and 10th October each year, because:

On 4th October 1957 the first human-made Earth satellite, Sputnik 1, was launched; and

On 10th October 1967: The Treaty on Principles Governing the Activities of States in the Exploration and Peaceful Uses of Outer Space, including the Moon and Other Celestial Bodies was signed ‚Äď see previous blog for more details.

This annual international celebration aims to inspire everyone about space, encourage young people to get involved in science, technology, engineering and maths and to demonstrate the benefits, and use, of space technology. The first World Space Week occurred in 2000, and each year has a specific theme.

2016 World Space Week
We‚Äôre really excited this year as the theme is ‚ÄėRemote Sensing: Enabling our Future‚Äô. It‚Äôs celebrating Earth Observation (EO), and highlighting the variety of EO missions in space and the applications which use their data.

There are over 1,000 events taking place all over the world to celebrate remote sensing, and they are all listed on the World Space Week website. It seems as though Brazil is holding the most events this year, a whopping 159! Have a look through and see if there is anything you‚Äôd like to go to. If not, create your own event ‚Äď

Spend a night looking at the stars.

Use Google Earth to look at your local area from space.

Get some friends together and watch classic space films.

Build your own spacecraft ‚Äď Both ESA and SSTL have cut out models you can use.

Competition!!

Competition Image courtesy of ESA.

Here at Pixalytics, we couldn‚Äôt let the Remote Sensing theme go by without getting involved. So we‚Äôve decided to run our first ever Twitter competition!! The prize is a copy of our book ‚ÄėPractical Handbook of Remote Sensing‚Äô, which guides complete beginners through the process of finding, downloading, analysing and applying remote sensing data. We‚Äôll post the book, free of charge, anywhere in the world!

The competition has now closed. Thanks to everyone who entered.

The location was Angkor Wat in Cambodia, read more about the site our next blog.

This image of Rio de Janeiro was acquired on the night of July 20, 2012 by the VIIRS instrument aboard the Suomi NPP satellite. Data courtesy of NASA/NASA‚Äôs Earth Observatory.

The Opening Ceremony of the Rio Olympics featured a plane taking off from the Maracan√£ Stadium and treating us to a fantastic night flight over Rio. It was a beautiful sequence to celebrate the famous Brazilian aviator Alberto Santos-Dumont, for us at Pixalytics it led to a conversation about the beauty of night-time satellite imagery!

Currently, the best source of night-time imagery comes from Visible Infrared Imaging Radiometer Suite (VIIRS) which is one of five instruments aboard the Suomi National Polar-orbiting Partnership satellite launched on 28 October 2011. Although, if you look on Twitter you‚Äôll also see a huge number of night-time images taken by astronauts aboard the International Space Station. This data has been used as the basis of the Cities at Night citizen science project whose aim is to create a Google maps style map of the world ‚Äď as the astronauts are using cameras to take photos of the places that interest them, and there is no georeferencing information, citizens identify the cities pictures.

In contrast VIIRS is an orbiting satellite and so continually collecting calibrated and georeferenced data of the whole globe. In the day VIIRS is collecting optical and temperature data over both the land and ocean, while at night it collects temperature data and the night-time imagery using the 750 m spatial resolution Day/Night Band (DNB). Working through both the night and day, the DNB needs to be calibrated through several orders of magnitude in brightness to accommodate the dramatic contrast between solar reflection and the darkness of night. Its forerunner was the uncalibrated Operational Linescan System (OLS) on the Defense Meteorological Satellite Program (DMSP) satellites, whose primary aim was to study clouds, but when its data was declassified in the 1970s it generated a lot of interest in low light night-time observations.

The DNB VIIRS images, like the one at the top of the blog, show hubs of human activity and the road arteries that connect them, and so are of special interest to the Campaign for the Protection of Rural England who use these types of maps to protect dark skies. It also enables calculations of light pollution to be made, together with indications of the associated carbon emissions. The DNB can pick up many different phenomena. For example, aurorae are visible, as well as gas flares, volcanic activity, the lights of ships, sea ice and climatological monitoring of clouds. It‚Äôs even possible to see thunderstorms, although individual lightning flashes are hard to make out in these snapshots, the glow inside clouds caused by them are evident as bright strips with DNB imagery as seen in this image from over Louisiana, USA on 4 April 2012 (Miller et al., 2013).

Another interesting discovery in 2012 was the presence of a faint ‚Äėnightglow‚Äô in the upper atmosphere on moonless night over the Pacific. The DNB team were aiming to collect scenes of complete darkness for calibration purposes, but they found clouds were still clearly visible. This was due to an assortment of photochemical reactions, especially of the molecule fragment hydroxyl, which allows this nightglow to pick up subtle atmospheric phenomena such as gravity waves and the tops of anvil clouds.

Here we‚Äôve gone from an aviation image inspired from 1903 to modern satellites, all via the Rio Olympics. It‚Äôs amazing where space can take you!

Blog written by Dr Louisa Reynolds and Andrew Lavender from Pixalytics Ltd.

Recent significant discoveries in Cambodia and Jordan have highlighted the potential offered by remote sensing and satellite imagery to help uncover secrets on Earth ‚Äď a field known as satellite archaeology.

Cambodia
Helicopter mounted Lidar was used to reveal multiple cities beneath the forest floor near the ancient temples of Angkor Wat in Cambodia. Lidar, which stands for Light Detection and Ranging, is an active optical remote sensing technique that uses a laser scanner to map the Earth‚Äôs topography by emitting a laser pulse and then receiving the backscattered signal. In Cambodia, a topographic Lidar with a near infrared laser was used by Australian archaeologist Dr Damian Evans to survey beneath the forest vegetation.

The conurbations discovered, surrounding the stone temple Preah Khan Kompong Svay, are believed to be between 900 to 1 400 years old. Analysis of the survey has shown a large number of homes packed together liked terraced houses, together with structures for managing water and geometric patterns formed from earth embankments ‚Äď which could be gardens.

At 734 square miles, the 2015 survey is also thought to be the most extensive of its type ever undertaken. Dr Evans work is due to be¬†published in the Journal of Archaeological Science.

Jordan
Archaeologists using high resolution satellite imagery, drones surveys and imagery within Google Earth have discovered a huge structure buried in the sand less than a kilometre south of the city of Petra. The two high resolution satellites used were Worldview-1 and Worldview-2, operated by DigitalGlobe. Worldview-1 was launched in September 2007 and has a half-metre panchromatic resolution; Worldview-2, launched two years later, offers similar panchromatic resolution and 1.85m multispectral resolution.

The outline of the structure measures 56m x 49m, and there is a smaller platform contained inside the larger one. Nearby pottery finds suggest the platform is 2 150 years old, and it is thought that it had a ceremonial purpose. The research undertaken by Sarah Parcak and Christopher Tuttle was published in the May 2016 edition of the Bulletin of the American Schools of Oriental Research.

Benefits of Remote Sensing & Satellites
Angkor Wat and Petra are both World Heritage sites, and the benefits of using remote sensing and satellite technology to undertake archaeological investigations are evident in the statement from¬†Christopher Tuttle who noted that they did not intend to excavate their Petra discovery as ‚ÄėThe moment you uncover something, it starts to disintegrate.‚Äô